1. Field of the Invention
This invention relates generally to computer memory systems and more particularly to a system and method for detecting a head positioning error within a magneto-optical computer memory device.
2. Description of the Background Art
Efficient and economic storage of digital information is an important consideration of manufacturers, designers and users of computing systems. In magneto-optical (MO) storage devices, digital data is typically stored in tracks located on rotating disks of MO storage media. Close positioning of the adjacent disk tracks maximizes the amount of stored data on a storage disk, thus providing significant economic benefits to system manufacturers and users. Therefore, system designers frequently seek new and improved methods of reducing track pitch to permit greater storage capacity on the storage media.
Referring now to FIG. 1, a plan view of a surface 102 of a MO storage medium 100 is shown. In MO storage devices, digital data is typically written to and read from a series of concentric or spiral tracks 104 located within a plurality of data wedges or sectors 106 on the surface 102 of storage medium 100. In practice, the digital data is typically read from surface 102 of storage media 100 by projecting a laser-generated light spot from a read/write head (hereinafter xe2x80x9cheadxe2x80x9d, and not shown in the figure) onto a selected track 104 while storage medium 100 is rotating, and then sensing the polarization of light reflected back from storage media 100.
It is critical for the head to be accurately positioned above track 104 of rotating storage medium 100 during a read/write operation on that track. Several factors (for example, imperfections in track symmetry or an off-axis wobble of the drive motor) may cause the head to be positioned slightly off the center of track 104, thus requiring position correction of the head to achieve satisfactory performance.
Various methods are known in the art for detecting and correcting mispositioning of the head. One well-known correction technique employs pre-patterned media having position marks formed on the tracks within a plurality of servo sectors 110 to generate a position signal. The position marks typically comprise uniformly shaped and sized concave depressions (pits) or convex protrusions (bumps) formed in surface 102 of MO storage medium 100 which reduce the local reflectivity, thereby effectively attenuating the light reflected back to the head. Generally, the position marks are grouped into a first and second set of marks, referred to respectively as the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d position marks. The xe2x80x9cAxe2x80x9d position marks are radially offset in a first direction from the track centerline by a predetermined distance. The xe2x80x9cBxe2x80x9d position marks are similarly offset from the track centerline by the same distance, but in a second direction opposite the xe2x80x9cAxe2x80x9d position marks.
Mispositioning of the head with respect to the track centerline is sensed as the light spot passes over the position marks and the head detects the amount of light reflected back. The resultant reflectivity waveform will include a first and a second set of pulses respectively corresponding to the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d position marks. The magnitudes of the first and second sets of pulses may then be separately determined to derive a first magnitude representative of the aggregate magnitude of the pulses caused by the xe2x80x9cAxe2x80x9d position marks, and a second magnitude representative of the aggregate magnitude of the pulses caused by the xe2x80x9cBxe2x80x9d position marks. The first and second magnitudes are then compared to determine if a mispositioning error exists. Specifically, equal magnitudes are indicative of proper head positioning, whereas an inequality denotes a positioning error (i.e., a larger first magnitude indicates that the head is offset from the track centerline in the direction of the xe2x80x9cAxe2x80x9d position marks, and a larger second magnitude indicates a mispositioning in the direction of the xe2x80x9cBxe2x80x9d position marks).
A disadvantage of the foregoing technique is that the reflectivity waveform will vary according to the radial positioning of the selected track. In particular, the pulse amplitude is substantially invariant with respect to the track position, but the pulse width corresponding to a track positioned relatively closer to the media center is greater than the pulse width corresponding to a track located relatively distant from the media center. The difference in pulse width results from the dependence of the local velocity on the radial position, i.e., since the rotational speed is constant, the local velocity at a given track will be proportional to the track radius. Thus, the time required to scan the light spot over a position mark of constant dimension will be a function of the track radial position, and the pulse widths will vary accordingly. A second, related problem of prior art techniques of the foregoing description is that the reflectivity waveforms have a DC offset component which will vary with the track radial position.
The variation of the reflectivity waveform with track position, as well as the presence of a DC offset, are undesirable and may complicate or reduce the accuracy of the positioning error sensing process. Thus, there is a need in the art for an improved head mispositioning detection technique that avoids these and other problems.
In accordance with the present invention, a system and method are disclosed for detecting mispositioning of a head device in a magneto-optical (MO) drive. The MO drive includes at least one rotating medium having a large number of spiral or concentric, closely spaced tracks along which data are written and read. A first and a second set of position marks are disposed along each track. The first and second sets of position marks are radially offset in opposite directions and are equally spaced from the track centerline. The individual position marks comprise optically detectable surface features, such as concave depressions (pits) or convex protrusions (bumps), of uniform shape and dimension.
The MO drive additionally includes a head device, which is positioned adjacent a selected track. The head device has a radiation source for directing a beam of light onto the selected track, and a detector for sensing light reflected from the selected track. The detector is configured to responsively generate an electric reflection signal, which includes a first and second set of pulses corresponding to the first and second sets of position marks. The reflection signal is passed to a differentiator, which differentiates the reflection signal to produce a first and second set of differentiated pulses respectively corresponding to the first and second set of position marks.
The differentiated reflection signal may then be conveyed to a low pass filter and second order resonator in order to remove undesirable low- and high-frequency noise components and thereby increase the signal-to-noise ratio. The filtered signal is thereafter passed to a finite time integrator, which determines a first area of the differentiated pulses corresponding to the first set of position marks, and a second area of the differentiated pulses corresponding to the second set of position marks. Differentiation of the pulses compensates for the variation of pulse width with local velocity, such that the pulse area is substantially invariant with respect to the track radius, as well as to other factors affecting the local velocity. In addition, inclusion of a differentiator in the detection path removes DC signal components that interfere with accurate determination of the area of the pulses.
An area analyzer conventionally detects mispositioning of the head device with respect to the track centerline by comparing the first and second areas determined by the integrator. In accordance with a preferred embodiment of the invention, sequentially numbered tracks are alternately provided with position marks disposed along the track centerline at either a xe2x80x9cCxe2x80x9d position or a xe2x80x9cDxe2x80x9d position. As is common in the art, a quadrature relationship between the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d position marks and the xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d position marks can be established thereby resulting in more reliable continuous determination of the head position.